1. Field of the Invention
The invention relates in general to a method for estimating a frequency difference, and more particularly to a method for estimating a frequency difference according to correlation results.
2. Description of the Related Art
Wideband Code Division Multiple Access (WCDMA) is a digital third-generation mobile communication technology.
In a WCDMA system, before a transmitter (a base station) transmits data, narrowband signals are first spread to broadband signals through spread spectrum coding and scrambled through scramble coding, and are then transmitted to a receiver (a cell phone). The data is in a unit of bits, and a spread sequence is in a unit of chips. As the chip rate during the transmission process in the WCMDA system is 3.84 Mcps, an actual bandwidth utilized by the WCMDA system is 3.84 MHz.
To allow the receiver to restore baseband signals transmitted by the transmitter, the cell phone first needs to synchronize with the base station, or else the baseband signals may not be properly restored by the cell phone due to unsynchronized timings of the receiver and the transmitter.
FIG. 1 shows schematic diagram of signal processing of a transmitter and a receiver in a WCDMA system. The left of the diagram shows a signal process at a base station, and the right of the diagram shows a signal process at a cell phone.
At the transmitter, a base station oscillator 106 generates a carrier signal. The baseband signal is up-converted through the carrier signal by a mixer 102 to generate a transmission signal. For identification purposes, in the description below, fb represents a frequency of the baseband signal, and fc represents a frequency of the carrier signal.
After the baseband signal having the frequency fb passes through the mixer 102, a mixed signal having a frequency fb±fc is generated. Through a filter (not shown), the frequency fb+fc or fb−fc of the mixed signal is selected as the frequency of a transmission signal. The transmission signal is transmitted from an antenna of the base station via a mobile communication network 10 and then received by an antenna of the cell phone.
When the transmission signal is received as a reception signal by the cell phone, a mixer 101 of the cell phone frequency shifts the reception signal by use of a reference oscillation signal generated by a local oscillator 105.
Theoretically, a frequency fref of the reference oscillation signal equals the frequency fc of the carrier signal, and so the baseband signal can be restored in intact from the reception signal. However, quite the contrary, the reception signal obtained after frequency shifting by use of the reference oscillation signal is different from the baseband signal initially transmitted from the transmitter. Apart from noises in the transmission process, the frequency fref of the reference oscillation signal is not entirely the same as the frequency fc of the carrier signal generated by the oscillator of the base station, and so a frequency difference ferror (ferror=frel−fc) exists between the two. The frequency difference ferror affects data contents obtained from demodulation at the cell phone.
According to WCDMA specifications, all base stations employ the same primary synchronization channel (PSCH) sequence, which is also pre-stored at the cell phone. In general, the receiver utilizes a PSCH correlator 103 to perform a correlation calculation on the PSCH sequence in the frequency-shifted signal to estimate the frequency difference according to the correlation result. Details of the PSCH correlator 103 are described with reference to FIGS. 2A and 2B shortly.
To determine the frequency difference ferror between the reference oscillation signal and the carrier signal, approaches for correcting the frequency of the reference oscillation signal are categorized into coarse correction and fine correction.
The coarse frequency correction is to reduce the frequency difference ferror to t within a frequency sweep step Δf by performing an initial correction on the frequency fref of the reference oscillation signal. After performing the coarse correction, a fine correction is performed on the frequency fref of the reference oscillation signal. The coarse frequency correction shall be discussed below.
Referring to FIG. 2A, a frequency sweep range is divided into a plurality of sweep frequencies fi, with every two sweep frequencies being spaced apart by a frequency sweep step. The frequency difference can be accordingly estimated.
As previously stated, the PSCH sequence is provided in advance by the WCDMA system. Thus, the known PSCH sequence is employed for estimating the frequency difference ferror between the frequency at the receiver (the frequency fref of the reference oscillation signal) and the frequency at the transmitter (the frequency fc of the carrier signal).
With a correlation calculation performed by a PSCH correlator, a value of the correlation result gets larger as the frequency difference between the frequency fref of the reference oscillation signal and the frequency fc of the carrier signal decreases. In contrast, the value of the correlation result gets smaller as the frequency difference between the frequency fref of the reference oscillation signal and the frequency fc of the carrier signal increases. Therefore, the receiver corrects the frequency fref of the reference oscillation signal generated by a local oscillator 105 by employing such characteristic and thus achieves the coarse frequency correction.
As shown in FIG. 2A, in a conventional approach for coarse frequency correction, a frequency sweep range is divided into multiple different sweep frequencies, and the sweep frequencies are sequentially utilized as the reference oscillation signal fref and tested.
It is assumed that the frequency sweep range is divided into 23 sweep frequencies, which are denoted by different numbers. That is, the sweep frequency fi represents an ith sweep frequency in the frequency sweep range. Besides, correlation results obtained from the reception signal which coordinating with the sweep frequencies and calculated by the PSCH correlator 103 are also numbered according to the corresponding sweep frequencies.
For example, a first correlation result y1 is obtained according to a first sweep frequency f1, a second correlation result y2 is obtained according to a second sweep frequency f2, and so forth.
Between every two sweep frequencies fi of the frequency sweep range is a predetermined frequency sweep step Δf. Selections regarding the frequency sweep range, the sweep frequency and the frequency sweep step may vary according to actual cell phone applications and system planning. Thus, only principles of the approach shall be discussed, whereas actual values selected and details of definitions shall be omitted.
After obtaining the correlation results yi according to the sweep frequencies fi in the frequency sweep range, a maximum correlation result ymax can be obtained by comparing all the correlation results yi.
Further, when the correlation result yi is the maximum value, the corresponding sweep frequency fi renders the frequency difference ferror to be a minimum value possibly yielded by the coarse frequency correction.
FIG. 2B shows a schematic diagram of acquiring the maximum value from the correlation results. After receiving the reception signal via the antenna, the reception signal is sampled and a correlation calculation is performed on the sampled data. In a conventional approach, a correlation calculation is performed on each of the sweep frequencies so that a corresponding peak value is obtained for each of the sweep frequencies.
By performing the correlation calculation on the sweep frequencies in FIG. 2A and recording peak values correspondingly calculated from the sweep frequencies, a waveform as shown in FIG. 2B is obtained. Assuming that the maximum value ymax corresponds to f13, it is estimated that the correlation result has a maximum value when the sweep frequency is approximately fi=f13.
As previously stated, the value of the correlation result is the largest when the frequency difference between the sweep frequency and the frequency of the carrier signal is the smallest, and the value of the correlation result gets smaller as the frequency difference increases. That is, according to the waveform in FIG. 2B, it can be deduced that, the frequency fref of the reference oscillation signal is most approximate to the frequency fc of the carrier signal when the frequency fref of the reference oscillation signal is the 13th sweep frequency f13.
Accordingly, in a conventional approach, the reference oscillation signal generated by the local oscillator 107 is corrected to the sweep frequency frel=f13 corresponding to the maximum correlation result.
In summary, in the above conventional approach, different sweep frequencies in a frequency sweep range are attempted as the frequency fref of the reference oscillation signal, and the sweep frequency corresponding to the maximum correlation result is identified therefrom. Such approach of testing and verifying the change in the sweep frequency fi one after another is utilized for coarse frequency correction of the reference oscillation signal.
FIG. 2C shows a schematic diagram of estimating a frequency difference according to two axes of time and frequency in the prior art.
According to WCDMA definitions, a signal is transmitted in a unit of frames having a length of 10 ms, with each frame having 15 slots. The slots of each frame are numbered from 1 to 15. Each of the slots contains 2560 chips. For example, slot 1 has chips 1 to 2560.
It is known from the above descriptions that, the frequency of the mixer needs to be repeatedly adjusted when sequentially utilizing different sweep frequencies as the frequency of the reference oscillation signal. A new receptions signal is then received by the antenna, followed by performing the correlation calculation using another sweep frequency. As the range of the frequency sweep range gets broader and the length of the reception signal gets longer, not only the number of attempts utilizing the reception signal becomes larger but also the process of the correlation calculation becomes longer.
Assuming that 23 sweep frequencies (f1, f2, . . . , and f23) are provided during the correction process, each set of sampled data is required to undergo 23 times of correlation calculation.
According to sampling principles, the sampled data needs to be at least twice of each chip. Since each slot includes 2560 chips, at least 5120 sets of sampled data are required based on the above sampling principles.
Moreover, tests utilizing 23 sweep frequencies need to be carried out for each set of the sampled data. It means that, for a reception signal having a length of one slot, 5120*23 times of correlation calculation are required.
However, the reception signal of one slot is insufficient for estimating the frequency difference. For example, to use 48 slots, 48*5120*23 times of correlation calculation are required. That is to say, in the above conventional approach, the times of correlation calculation significantly expands as the number of slots utilized increases.
Therefore, assuming that the reception signal required for the frequency correction is ΔT, the time for processing the reception signal is negligible and an L number of sweep frequencies are sequentially scanned in turn, the above approach needs a processing time of substantially ΔT*L. It can be concluded that, the above approach of repeatedly performing the correlation calculation is rather inefficient. Further, in situations of a fresh boot, out of system services and searching for a network in a background during roaming, the above approach consumes an immense amount of power during the process of cell search of the cell phone.
FIG. 3 shows a schematic diagram of an accelerated frequency difference estimation of the prior art. In simple, a reception signal is first converted to a digital format and stored in a memory, and the digitized data stored in the memory is then frequency-shifted by a digital approach.
An analog-to-digital converter (ADC) 201 first converts a reception signal from an analog format to a digital format. The reception signal in a digital format is then stored by a data storage unit 209.
Since the reception signal is stored in the data storage unit 209 in a digital format, frequency shifting and correlation calculation may be directly performed on the reception signal in a digital format by a digital approach in subsequent operations. That is to say, in FIG. 3B, a mixer 202 for frequency shifting, a PSCH correlator 203, a weighted accumulation calculation 205, and a determiner 207 are practiced in a digital domain.
Similarly, it is assumed that, to estimate the frequency difference of a reception signal having a length of 48 slots utilizing 23 sweep frequencies, a time length of 48 slots is needed for storing the reception signal. Without repeated reception processes of the base station, 48*5120*23 times of the subsequent correlation calculation are performed in the cell phone, so that the required time is considerably reduced.
For an N number of slots, assuming that the total time spent on the frequency shifting and correlation calculation is ΔS, the total time required by the approach in FIG. 3 is substantially ΔT+ΔS.
With the conventional solution implemented by a digital approach shown in FIG. 3, as the frequency shifting, PSCH correlation calculation, weighted accumulation calculation and frequency difference estimation are all performed in the digital domain, an overall processing time is remarkably reduced.
However, although the above approach of record-and-replay offers a reduced time of frequency difference estimation, a data storage unit 209 having a huge capacity is required at the receiver.
Taking a reception signal having a length of 48 slots for example, a total of 48*5120 sets of sampled data need to be recorded. When processing the sampled data, the sampled data are divided into an I-branch and a Q-branch. Assuming that either of the I-branch and Q-branch is 8-bit in length, the data storage unit 209 demands a buffer size of 48*5128*(8+8) bits.
Further, the above approach, before repeatedly playing different sweep frequencies, needs to record the frequency-shifted signals (i.e., initial frequency-shifted signals) generated from frequency shifting the reference oscillation signal into a buffer. In addition, to yield even more accurate frequency difference estimation, the approach in FIG. 3 also demands a storage unit 209 having an even larger capacity. Thus, although such approach accelerates the processing speed, hardware costs may be greatly increased by the memory space for storing the additional chips.
FIG. 4 shows a schematic diagram of processing time and storage space required for both of the above conventional solutions. From FIG. 4, the architecture in FIG. 1 consumes an enormous processing time although no additional storage spaces is required. In contrast, the architecture in FIG. 3B requires a greatly increased storage space despite that the processing time is shorter compared to that of FIG. 1.
From the above descriptions of the prior art, it is concluded that, the conventional approaches either needs a lengthy processing time or a storage space having an enormous capacity. Therefore, there is a need for a solution that satisfies both processing time and hardware cost requirements.